Wave translating system



Oct. 7, 1941.

WAVE TRANSLA'I' ING Filed Deo. 20,

H. S. BLACK SYSTEM 2 Sheets-Sheet l ATToR/vzfgl Oct. 7, 1941. H. s. BLACK WAVE TRANSLATING SYSTEM 2 Sheets-Sheet 2 Filed Deo GRID 0F FIRST TUBE SIL VER THER'mLLr sms/r Res/s SUL PH/DE um rre/Vasa Repu 751? /02I UNA T TENDE 0 PE4 TER A T TEA/DED S T4 TION NVENTOR H sLAc/r Y BV ATTORNEY Patented Oct. 7, 1941 2;,

WAVETRANSLATINGIS'YSTEM- f' v Harold s. Black, ElmhurstgN. Y., assignent() Bell :Telephone Laboratories,

Incorporated, New

York, N. Y., a corporatie-noi N ew York y q Application December 20, i938, serial No. 246,791 .7.

' ifclarhs. 101.5250436),

This invention relates to wave translating systems, as for example, oscillators andoscillatoramplifiers.

` An oscillator may be regarded as an amplifier plus'aretroactive coupling or oscillation; circuit whose'critic'al phase angle determinesV the oscillation frequency (the free response of the system beingan expanding waveand the amplitude Vof' the oscillations being limited 'by non-linearity of the' amplifying device or some other elementy ofthe system). In adding a retroactive coupling or oscillation circuit of critical phase frequency characteristic to'an amplifier to, form an oscillator, the optimum phase shift for the amplifier isV the negative of the phase shift of the oscillation circuit at its critical frequency, at which its frequency stabilizing effect vis a maximum. For example, With'the usual bridge or other'types of coupling circuit having phase shift 1'80;,de grecs at the critical frequency, an amplifier phase shift of 180 degrees enables'the coupling.

circuit to operate at its critical frequency, at which its frequency stabilizing effect vis a maximum; and such a system will not oscillatea-t all if the phase angle of the ampliiierjisqinthe rst or fourth' quadrants. v

VAn object of the inventionis to facilitate ob;- taining the optimum phase :shift-'in an amplifier or. driving circuit for an oscillator orwave generator. Y f v In one specic aspect-the invention is an oscillator in which the optimum amplifier phase shift for the oscillation frequency is obtained by negative feedback in 'the amplifier at-that frequency. VFor example, 'the oscillator may be of the general' type (disclosed, 'for instance, sin L. A. Meacham Patent 2,163,403, -or inthe paper by L. A. Meacham on Thef Bridge Stabilized Oscillator, VProceedings of the Institute Y of Radio Engineers, October 1938), whosefrequency determining circuit coupling the amplifier-output terminalsl to the amplifier input terminals is a bridge network with 180 degrees phaseshift',y at

the desired oscillation frequency,` and in accordance with the invention the amplifier may include a negative feedback path conjugate to the coupling circuit to render the amplifier phase shift 180 degrees at that frequency. For instance, with the amount of negative feedback in the amplifier sufficient, for example of the order of upwards of twenty decibels, the `amplifier phase shift can be made substantially 180 degrees by making the phase shift of the feedback path substantially zero; for, as explained in my Patent 2,102,611, the phase shift. of an amplifier withv large negative v'feedbackV approaches 180 degrees minus` that of the feedback path, for example the phase shift of such an amplifier approaching 180 degrees if the phase shift of the feedback path is zero. v

For best frequency stability it is desirable not only to operate with the mean value of the phase shift of the bridge ynetwork or coupling circuit Vand the mean` value of the phase shift ofthe ampliiernearfto 180 degrees (in order to minimize the effect upon theoscillator frequency of any change `in the amplifier gain or phase shift `Aor a `change in a bridge resistance), but also to keep the phase variations ofthe amplifier and the gain variations of the amplifier as small as4 possible, and, moreover, to have ,the amplifier gain high (the higher the gain, the higher. the

frequency stability, other things being equal).

AFixing the amplifier phase..shift at 180degrees byra largevalue of negativefeedback Yfacilitates obtainingall of jthese, desirable operating conditions.Y It reduces the amplifier phase Ashift variations; Vand lit gives-,flexibility as to the number of" amplifier stages ',(and interstage circuits) available and thus facilitates giving the amplifier (aswell as the amplifying element .or-device .of the amplifier) yhigh gainnwhile maintaining the 180 degree phase shift of theampliii'er.

. @If desired, the amplitude of the oscillations Imay-be limited to and stabilized at a value below the overload value for the amplifying element.

vkThis may be done, for instance, by vhaving an vamplitude stabilizing element in the oscillationproducing coupling circuit in the general manner disclosed in R. S. Caruthers Patent 2,066,333, January 5, 1-937, and the Meacham patent and paper mentioned above. For example, the amplitude stabilizing element may vary its impedance, in response to variations of the oscillation amplitude, so as to vary the attenuation of the coupling circuit in such manner as to maintain the gain forpropagation once'around the oscillator loopy zero decibels at an oscillation amplitudethat is below the overloadvalue for the amplifying element and is substantially constant at the amplifier output,` and thus maintain the Voscillationy amplitude substantially constant at the amplifier output. Further, the amplitude yoffthel oscillations may thus belimited to a value far below the overload `valueifor the amplifying incoming circuit and transmitting the amplined waves to an outgoing circuit, for example in the general fashion disclosed in the Caruthers patent.

If desired the oscillator, or the oscillator-amplier, may have a plurality of oscillation or frequency determining circuits coupling the amplifier output terminals to its input terminals, each coupling circuit having a suitable amplitude stabilizing element for limiting the oscillation amplitude as indicated above, so that the system produces oscillations of a plurality of frequencies,

which need not be related to each other but, as long as they differ substantially fromI the frequencies to be amplified by the system, ymay have any desired values, even values that are close to frequencies to be amplified.

The amplitude stabilizing element may be of any suitable type. For example, it may be the Thyrite, copper oxide rectifier, or space discharge device referred to in the Caruthers patent as exhibiting an overload or non-linear relation between instantaneous values of current and voltage; -or it may be variable with its temperature in response to change in magnitude of the current traversing it. In accordance with the invention, the amplitude stabilizing element may preferably be a so-called thermister, or temperature dependent, non-linear resistance element of the general type that varies the magnitude of its resistance enormously in response to change of its temperature, so as to exhibit a non-linear relation of current to voltage when heated by the current passing through it. For example, the

`non-linear element may be a resistance of silver sulphide, uranium oxide, cobalt oxide, or boron, with its operating and restoring times (i. e., the times required for it to heat and cool) less than the times required for the oscillations to buildup and decay, a sufficient condition to insure against any tendency for the oscillations to build up in spurts when the system is switched or keyed to the condition for oscillation production, yet with the time required to alter the thermister temperature (and hence to alter the thermister resistance) large compared to the time required for execution of changes in instantaneous current or voltage at any frequency falling within the useful range of frequencies of the amplifier or the oscillator, thus avoiding production of deleterious modulation by the amplitude limiting or amplitude stabilizing element.

Other objects andfeatures'of the invention will be apparent from the following description and claims. l

Fig. 1 is a circuit diagram of a self-oscillating feedback amplifier, or wave generating and wave amplifying system embodying oneform of the invention;

Fig. 2 is a circuit diagram 0f a modification of the self-oscillating amplifier of Fig. 1 which .produces oscillations of a plurality of frequencies;

Fig. 3 is a schematic circuit diagram of a system including the amplifier of Fig. 2; and

Fig. Llis a circuit diagram of a modification of theamplifier of Fig.`2.

The system of Fig. 1 comprises an amplifier 5 having input terminals I and 2 and output terminals 3 and 4, and comprises a coupling circuit 6 coupling the amplifier' output and input circuits for generating sustained oscillations ofgiven frequency, and also comprises an incoming circuit I with input transformer I2 for supplying the amplier with waves to be amplified and an outgoing circuit or load circuit II with output transdesired value.

former I3 for utilizing the amplified waves and the generated oscillations.

The waves to be amplified may be, for example, a wide frequency band of signal waves. For instance, these waves may be speech modulated carrier waves of a number of carrier telephone channels of a multiplex carrier telephone system.

The frequency of the generated oscillations, so long as it differs substantially from the frequencies of the waves to be amplified, may have any The generated oscillations or waves may be utilized for any desired purposes, a few examples being given hereinafter.

Theamplifier 5 comprises an amplifying path including an amplifying element or device I5 and a feedback path I6 connecting the output and input circuits of the amplifier for producing negative feedback in the amplifier.

The device I5 may be, for example, a vacuum tube amplifying device having a single vacuum tube Astage or any desired number of cascaded stages of triodes, pentodes, or other suitable vacuum tubes, as indicated by the legends applied to the grid and plate shown in the drawing. Grid biasing potential may be supplied in any suitable manner, as indicated, for example, by a resistor I 8 in the cathode lead which is traversed by plate current and which is by-passed for alternating current by condenser I9. 'I'he grid bias for the tube or tubes of the amplifier preferably is sufliciently large to'prevent flow of grid current dur- 'ing normal operation of the system.

An input bridge 20 connects the terminals I and 2 to the input of theamplifying device and connects the feedback path I6 to the input of the amplifying device, giving conjugacy between the feedback path I6 and the circuits attached to the amplifier at its input terminals I and 2, i. e., the incoming circuit I0 Yof the system and the oscillator coupling circuit 6.

An output bridge 30 connects output of the amplifying device to the terminals 3 and 4 and connects the output of the amplifying device to the feedback path I6, giving conjugacy between the-feedback path I6 and the circuits atached to the amplifier at its output terminals 3 and 4,

i. e., the outgoing or load circuit I I and the oscillator coupling circuit 6.

The feedback path I6 includes a stopping condenser of negligible reactance for the frequencies of the waves to be amplified and the oscillations tobe generated.

An input bridge connects the incoming circuit I0 to the amplifier input terminals I and 2 and connects the oscillator coupling circuit 6 to the amplifier input terminals I and 2, giving conjugacy between the oscillator coupling circuit 6 and the incoming circuit I0.

An output bridge 5D connects the amplifier output terminals 3 and 4 to the outgoing circuit I I and connects the amplifier output terminals 3 and 4 to the oscillator coupling circuit 6, giving conjugacy between the oscillator coupling circuit 6 and the load circuit I I.

The oscillator coupling circuit 6 comprises a frequency stabilizing bridge network 60, amplitude limiting or amplitude stabilizing means Rr, a transformer 'l0 which prevents short-circuiting of one vof the arms of bridge G6, and a frequency selective network l5. The network 15 may be a circuit tuned to the frequency of the oscillations to-be generated, and serves to prevent transmission from bridge to amplitude limiter vRT of waves of other frequencies, espeseries, the resistance of inductance 82 being considered as included in resistance 81|. Inducta'nce 82 and capacity B3 resonate at the desired oscillator frequency. Rsistances R5, Rs, R7 and 8l preferably have approximately equal values, so that at the resonance frequency the bridgeBD is balanced except for a` slight resistive unbalance.

The amplitude stabilizing element RT may be an impedance variable in magnitude with the strength of the current traversing it. For example, it may be a Thyrite element or copper oxide rectier or space discharge device exhibiting an overload or non-linearcharacteristic as referred to above. Preferably, it is a temperature Q dependent non-linear resistance, for example a thermister unit of silver sulphide, uranium oxide,

cobalt oxide or boron, varying its impedance in response to variations of the oscillation amplitude, so as to vary'the attenuation of the coupling circuit in such manner as to maintain the oscillation amplitude at the amplifier output substantially constant at a value below the overload value for the amplifier, as indicated above. The thermister may have its operating and restoring times less than the times required for the oscillations to build up and decay, yet sufficient to avoid production of .deleterious modulation, as indicated above. Such condition is readily obtainable in the system shown, for two reasons. One reason is that, with the frequency determining circuit in the bridgeas shown and with high ampliiication in the amplifier, the frequency selective bridge circuit is equivalent toV a series-resonant circuit of very high effective Q lconnected across ythe coupling circuit'. (In general, the narrower thev` pass band o r frequency band of response of the paththrough the amplifier and coupling circuit, the longer are the times taken for the oscillations tobuild up and decay.

In the system shown, these times tend to increase as the gain of the amplifier is increased. Moreover, they can bev increased by reducing the resistances of bridge 6%),reducing the impedances viewed from the .bridge toward 10 and toward RT, and increasing the inductanceBZ.) The second reason is that the operate and release times of the thermister can be vmade to vapproach equality and also can be varied-through very wide limits by the mechanicaland thermal design of the thermister units., For example, the release time is ameasure of the heat loss of the thermister and in a well designed assembly will be largely controlled bythe mass of the unit and its vheat loss, which is largely due to heat conduction along its two lead wires.v If the operate time is materially shorter thanthe release time (it is' never. longer) additional mass can be'placed in thermal but not electrical contact with the surface of the thermister. This lengthens the operate time and shortens the release time and can be continued until the two arealmost equal with some attendant loss in sensitivity. The smaller the mass of the thermister; the shorter the operate and release times. Units l' housing the thermister ina heat'insulated room.

Thus; placing the thermister' in an insulated chamber; s'uch as 21, suiiiciently large with regardfto the thermisterunitit'self and the power it d'issipates, greatlyreducesthe effect upon the Ythermist'er 'of iiuctuations in'lroom temperature and also materially reduces itsr slow temperature changes, yet does not materially effect its relative times of heating and cooling.

Where requiredbythe desired'pre'cision of operation of the thermister, objectionable elects of 'room temperaturevariations canbe obviated by maintainingrthe heatinsulated chamber 21 at a fixed temperature, 'in excess of the highest room temperature. For example, vin the general manner disclosedin; my Patent No. 2,209,955, August 6, 1940, for Wave translation system, this can be accomplished by-a chamber heating element 28 supplied with heating current from a power source through' regulating network N and conductors 36V. Thisv network has a series resistance arm 3l and a shunt arm comprising a resistance 32 Vin series With'two parallel resistances 33 and 34;' resistance 433 being-a resistance with a negativev4 temperature coefficient, as for example, a silver sulphide thermister. "The constants of thenetwork depend upon thethermal and other properties of the particular unit 33. With the temperature, of the 'chamberl'21 elevated above the highest -roo'm temperature by the chamber heatingelement 28, iftheir-room temperature rises the'resistance of the-unit 433 falls sufficientlyv to reduce the heating current in -the .element y28 so that the temperature of the chamber 21 is held cc'iristant. The source should be Aa constant voltagel or current source; preferably asfnearly constantasy practicable. In accordance with the invention it vcan comprise a""cc' i'l'l29` inductively coupled to a -coil 31 tuned to' Ythe'o'scillation frequency by condenser v38, sovthat the oscillations generated by the systemP supply the power for network N and heater 28. Since the power required Afor vthis 'pur'pose'can be made trivial, these oscillations are especially well adapted for this purpose Aas they are of more nearly'co'nsta'nt magnitudel than even a lwell-regulatedk ordinary power supply.

In operation of the system, kWaves to be vamplied are transmitted from circuit l0 through bridge 40 to the amplifier terminals I and 2, the amplifier input impedanceviewed to the right from terminals I and 2 being determined by three of the four arms of the bridge 40. The waves amplified by the negative feedback amplier 5 'pass from the amplifier output terminals 3 and 4 vthrough bridge 50 to the load circuit Il, being excluded from the oscillator coupling by the tuned circuit 15. VThe amplifier output impedance4 viewed to the left from terminals 3 and 4 is determined by three arms of bridge 50.

Regardless of whether waves are thus being supplied from line l0 for amplification, the system can generate oscillations stabilized as to frequency and amplitude, oscillations from the amplifier output terminals 3 'and 4 being transmitted through bridge 50, transformerl and tuned circuit 'I5 and impressed across one pair of the diagonally opposite corners B and D of the bridge 60, and the unbalance potential, appearing across the other pair A and C, being transmitted through the circuit comprising thermister RT and bridge 4U to the amplifier input terminals I and 2. Assuming that at the oscillation frequency the phase shift of the amplifier 5 is 180 degrees and the phase shift through transformer 'I0 is negligibly small, or that the phase shift through the portion of the oscillator loop external to the bridge 60 is 180 degrees, the requisite phase shift through the coupling circuit 6 for the production of oscillations may be obtained by poling the connections of transformer "I0, or by interchanging the connections of the corners of bridge 60.

The bridge 60 is kept as nearly in .exact balance as practicable, but order that the circuit may oscillate a slight -unbalance is required, which, however, can be very slight provided the gain of amplier 5 is high. Accordingly, one of the resistances of the bridge may have a value slightly different from the others. For example, resistance R7 may beslightly greater or less than each of the other three resistances (according to whether the greater or the lesser value is required to give the phase shift through the bridge the proper sign for production of oscillations), so that the attenuation through the coupling circuit 6 including the bridge 60 and the thermister Rr is just equal to the gain-of the amplifier 5.

This attenuation is controlled by the thermister in response to change in the oscillation amplitude. When oscillations are initiated, the thermister is cold and its resistance is large. Thus the attenuation produced by the thermister in transmission through the circuit 6 is small and oscillation amplitude increases rapidly. As the oscillating current traversing the thermister heats it, its resistance decreases, approaching the value for which the loss through the coupling 6 equals the gain through the amplifier v5, and reaching that value when the oscillation amplitude is well below the overload value for amplifier 5. Should the gain of the amplifier then change, an extremely smallconsequent change in the amplitude of the oscillations at the amplifier output changes the temperature of the rthermister sufficiently and in proper sense to so change the attenuation of the circuit 6 as to yrender the loss in the circuit 6 equal to the gain in amplifier 5. This automatic adjustment stabilizes the oscillation amplitude at the amplifier output.

The frequency control exerted by the bridge 60 depends upon the fact that at the actual oscillation frequency the phase shift of the amplifier 5 must be equal and opposite to that through the circuit 6, as explained, for example, in the abovementioned Meacham patent and paper. The phase angle between the input and output voltages of the bridge 60 is dependent on the resistance unbalance of the bridge.- When the degree of resistance unbalance is small this phase angle is Very strongly variable with the reactance of the frequency selective branch at frequencies close to resonance. A relatively large phase shift produced by elements external to the bridge 60 can therefore -be compensated by the phase shift in the bridge itself resulting from an extremely small departure of the frequency from the resonance value. The operation of the bridge in a nearly balanced condition, therefore, increases the stability of the ,oscillation frequency. When the amplifier` 5 changes its vphase shift for some reason, even by an amount far in excess of what would be anticipated in practice, an extremely small consequent change in the phase angle (between the voltage and the current) of the impedance Z included in bridge suffices to change the phase of circuit 6 sufficiently and in proper sense to maintain the phase of circuit 6 equal and opposite to that of amplifier 5. That is, the tuned circuit comprising inductance 82 and capacity 83 still operates very -close to resonance in spite of the exaggerated change in the amplifier phase shift. The greater the amplifier gain (with other conditions unchanged), the greater is the frequency stabilizing effect of the bridge 60.

The amount of negative feedback through the path I6 may be large for the frequencies supplied to the amplifier 5 from circuit I0 and also for the oscillation frequency and any harmonics thereof that it is desired to suppress by the feedback. For example, the reduction in gain of amplifier 5 effected by that feedback may be of the order of one or many more times ten decibels for those frequencies. This feedback can greatly facilitate obtaining the optimum yamplifier phase shift for production of the stabilized oscillation frequency. For example, the amplifier 5 may be a high gain amplifier with a plurality of interstage coupling circuits, for amplifying a broad band of waves supplied from circuit I0, as well as waves of the oscillation frequency, the phase shift through the amplifier 5 without the negative feedback through path I6 being substantially different from 180 degrees at the desired oscillator frequency; and the negative feedback path I6 may have zero phase shift at thatfrequency, to make the amplifier phase shift with the negative feedback 180 degrees from terminals I and 2 to terminals 3 .and 4.

path I6.

For example, if it is desired to take into account phase shift of transformer 1B, a duplicate of the transformer can be connected in path I6 and the phase shift of path I6 as so modified can .be made equal to that of transformer 10, at the desired oscillator frequency. Then, with path I6 producing sufficient negative feedback in the amplifier 5 at that frequency, the amplifier phase shift will be the negative of degrees plus the phase shift of transformer 10).

The negative feedback through path I6 is advantageous for suppressing harmonics of the oscillation frequency that may be generated in the amplifier, due for instance to non-linearity of the characteristic of an amplifying element. The selectivity of the tuned rejector circuit 'I5 prevents these harmonics from being suppressed by negative feedback through the ,coupling circuit 6. Need for circuit 'I5 in the self-oscillating amplifier arises from the use of a frequency determining circuit such as 60 `of bridge or balancing type or type which has vits selectivity effectively enhanced by the amplification of amplifier 5; for in case it is permissible to use, instead, a frequency determining circuit giving a lower effective Qf for-instance'a paralleltuned circuit connected in shunt Ato thermisteri in the gen^ eral fashion in'whichatuned frequency deter-Y mining circuit is shown connected across the oscillator coupling circuit in the above-mentioned Caruthers patent, the circuit '|51v then ordinarilyl can be omitted. Bridges 20 and 30', assuming ,u 1, provide excellent impedances for the fourth arm'of'bridges 4Il`andv50 and as a'result the A'amplitude and :frequency of the oscillations will beto a veryhighl degree-.independent of-the impedances that lmay beconnected to I and II. In ya similar manner thevv over-al1 amplification from I9 to II is independent. of. couplingl'circuits. f

l The cathodes of the amplifier may be heated in any suitable manner, for example, in the man-V the am.

ner shown and described hereinafter for pliler of Fig, 2, if desired.;

omitting the amplitude limiting lment R55 and the tuned circuit 'I5 (forexample byopening a switch, not showngconnectedin series with element RT and closing a switch; not-shown,

across and adjusting* the bridge 6B to` re A sistance balance at the resonance frequency of' impedance Z, the system becomesa highly selective amplifier, transmitting from line I0 to line II waves of that frequency to the exclusion of other frequencies. The repetition of rthe feedback process, provided bythe conjugate feedback pathsfthen Venhances the gain stability and the i frequency selectivity ofthe amplifier. It is not necessary that the bridge v-bejin exact resistance balance at the frequencygof peak gain of the selective amplifier. An improvement in-gain stability of the amplifier with-circuit constants can berealized in the region of peak gain with the bridge adjusted to give a small ramount of negativeor positive feedback at the frequency of peak gain, as for example 4 or 5 decibels of negative feedback or one-half decibelvof positive feedback.-

circuits 6AV and 6B,V each comprising elements'.

structurally and functionally similar to thoselof the coupling circuit Ii of Fig. 1, the frequency` determining circuits in bridge armsZ and Z of the coupling circuits 6A and 6B, respectively, bee

ing tuned to the desired oscillation frequencies.

Portions of the coupling circuits may be common,

the bridge 50 and transformer 'I0 being so shown.

However, the coupling circuit 6A has its 'individual frequency stabilizing circuit 60, rejector circuit I5 and amplitude stabilizing thermister RT corresponding to those of Fig. 1, and the coupling circuit SB has respectively corresponding individual frequency stabilizing circuit Vfill', rejectorA circuit 'I5' and amplitude stabilizing ther-` mister RT. The circuit 1 5 is tuned tothe resonance frequency of inductance 82' and capacity 83'. Portions of fcoupling circuit 6B respectively corresponding to portions of coupling circuit 6 areidesignated by the'sarne reference characters primed.. VThe bridgeA shown in Fig. 1 is dis'- The self-oscillating amplifier kof Fig. 2 is simipensed with inthe system of Fig. 2, thecoupling A of'.1 watt.

circuits lAY and GB beingshown in series with the incoming circuit I0.

TheV oscillation-frequencies may lie as close to the frequency band of the waves supplied by circuit I0 as is` permitted by the ability of the rejector circuits 'I5 and 'I5' to prevent those waves from unduly interfering with the amplitude stabilizing functions of* the thermistersR'i` and RTlf-*Any desired "number of coupling circuits such as 6A and 6B may be employed, to give oscillations of `variousfrequencies stabilized as to frequency and amplitude.

When the self-oscillating amplifier is an ampli-r fier of an unattended repeatervfor a cable Containing lines I0 and I I, it is economical to transmit'the power forthe amplifier over the cable. It is desirable to transmit direct current instead of alternating current because the line loss is less, direct current does not interfere into other circuits, more power can be transmitted for a given allowabley line'voltage, and a storage battery can ServeiA as a standby without switching. The B battery voltage can readily be transmitted ben causevthis voltage is high and the plate current low. :-Even small tubes are available'whcse fila mentfpower vconsumption is less than the'plate powerjfor example, "a pentode having a filamen-v tary'cathodewith a plate current of 5 milliamperes at 300 volts: watts and a ilamentpowe'r The tube ortub'es of amplifier 5 may be of Vsuch type. The'output of the last tube can be adjusted to something in excess of the amount of power :requireda 'to heat the tube cathodes,

(while reserving adequate load carrying capacityV VYof' the'amplifier for the waves from circuit I0) put transformer, and supplied to the amplifierV filaments' for heating-them as indicated in the drawing. AFor starting the oscillations, it sufce's to-preheat the filaments with a battery 81 and then operate switch 88 (in less time than the flla mentwould require to cool), to connect the fila ments vto the low voltage side ofthe transformer,

(which canbe an inexpensive one when'the oscil lation `frequency is high) The" amplifier 'will then-.work under its -own- B Vbatteryl power as long as requirement is met that-this power be not interrupted.. (This 1is, a practical requirement, met. by present B battery plants in order to avoid p'roducing't'elegraphhits on existing voice andA carrier', .sy/steunt.) With theY oscillations thus 551 heatingV the' ar'npliier,filaments,v the amplier is Y still", capable Yfof delivering considerable undistortedpower, when Voperated class A.V4 -Condensers 89 and 90 are shown for tuning the-.windings of ,transforme r ,85. Incase .the ampliiier'has filaments requiring different.' voltages, .these may f be.V supplied .from different secondary v-windings Y that may beprovided ontransformer-85, if de-` sired.

example -as operated in parallel with each other from the same tuned transformer. 31,29, network Naud conductorsv 36.v f The kcondenser' 38 ,can

"erationfof'fth self-oscillating amplifier, as fory gInFig. 2 the-heaters 21B-and fffor the ampli` l tudeflimiting thermistersare indicated by wayof instance for the purpose of enabling the amplifier to effect self-testing of its modulation. For example, two weak (compared tothe full output of the amplifier) oscillations can be produced by the oscillator and maintained at two closely spaced frequencies outside of the band of the amplifier and preferably above so as not to reach the next repeater. Their difference frequency will also be Ya low frequency not transmitted by the output transformer and also not subject to much feedback. In this way the difference frequency current will be a measure of the modulation without feedback. vIt can be selected by a tuned transformer 9| having a primary winding in series with the output transformer and if in excess of tolerable limits will be caused, through an amplifier 92 if desired, to vary the resistance of a sensitive thermister 93, by passage through the thermister. This thermister, in series with resistances 94, is bridged, by means of conductors 95, across a fine gauge wire pair v 96 to transmit this information back to an attended station. This thermister 93 may beone such as the thermister RT, for example of silver sulphide, and may have a heater 91 (like the heater 28 of thermister Rr), supplied With oscillations from conductors 36 for maintaining the ambient temperature of the thermister fixed at a value in excess of the highest room temperature, to obviate objectionable eiects of room temperature Variations upon the operation of the thermister. suitable type. If of thermionic type, it may have its cathodes heated, for example, from a tuned secondary winding that may be added to transformer 85 for the purpose, in the fashion indicated for the filaments of amplifier 5.

Fig. 3 shows repeaters IUI and 102 connecting line sections such as Ill and Il. Each of these repeaters contains an amplifier such as that of Fig. 2. Fig. 3 shows the conductors 95 that connect the pair of conductors 96v with the thermister 93 in the amplifier, and shows the pair 9'6 exten-ding back to an attended station or terminal |05.

At this station, measurement or indication of the` resistance viewed when looking into the pair amplifiers that may have thus operated their faulty repeater maybe eifectedmanually or automatically in any suitable manner, as, for example in the general'fashion of impedan'cemeasl-` urements and fault locations for long cable 'or other circuits.

Figfl shows a self-oscillating amplifier' like that of Fig. 2 except that'l the filament heating.

circuit (which may be like that of Fig. 2) is omitted for simplicity, the bridge 40 is included (as. in Fig. 1),y and coupling circuits 6G and 6D which correspond respectively to circuits 6A 65 and :BB of Fig. 2, have frequency stabilizingand,v

amplitude stabilizing circuits 60C and BUD-that Amplifier 92, if used, may be of any aresomewhat differentfrornthose of the coupling; ,Y

circuits 6A and '6B and that `are connected to bridgeY in parallel With each other, The cir-- cuit C 'is'a frequency selective bridged T network'of the type shown in Fig. .8` of` the abovementioned Meacham patent. *Ther T., network comprisesequal series branchesof capacity C, the shuntbranch, consists Yof aninductance 'L2 anda resistance R2 in series, and the bridging impedance is a variable resistor R1 which is an amplitude control element for limiting and stabilizing` the amplitude of the oscillations produced by the feedback through the coupling circuit 6C. As pointed out in the Meacham patent just nientioned, Ri should be somewhat greater than 4R2 and for control'purposes should have a negative temperature coefficient. The oscillation frequency lies Very'close to the value `determined bythe product 2L2C. Stability of the oscillation frequency is obtained by making the difference betweenRi and 4R2 very small, that is, by making the attenuation of the circuit 60C Very great atA the oscillation frequency, and is enhanced by making the capacity C small and the inductance L2 large. The resistor Ri may be, for example, a thermister of the type of the thermister RT of Fig. -1, with its operate and release times, or heating and cooling times, preferably adjusted as describedabove for thermister RT.

.'The rejecting circuits 15 of Fig. 4 correspond to the rejecting circuit 15 of Figs. 1 and 2, and are tuned to the oscillation frequency determined by thevcircuit 60C.

The coupling circuit 60D is like the circuit C except adjusted to produce a different oscillation frequency. Portions of circuit 60D respectively corresponding to portions of circuit GC are designated by the same reference characters primed. Any desired number of coupling circuitsv such as '6C and 6D may be employed, to give oscillations of various frequencies stabilized as to frequency and amplitude.

What is claimed is: Y

1. A wave translating system comprising an amplifying device and conjugate feedback paths therefor, one forming with said device an ampli*- er havingl negative feedback that renders the phase shiftof said amplier substantially degrees at a given frequency and another forming .with said amplifier a circuit producing selfsustained oscillations of said frequency. Y

2. An oscillator comprising an amplifier hav ing input terminals and output terminals and having a retroactive coupling circuit for producing sustained oscillations Yof givenfrequency connected between said output terminals and said input terminals, said coupling circuit including a. frequency stabilizing bridge networkV with 180 degreesphase shiftat said given frequency, and saidilamplier'including a feedback path and balancingcircuits rendering saidpath conjugate to said Coupling circuit and thereby preventing said:A couplingci-r-cuit from affecting Ithe amplifier phase shift,said pathr having substantially zero,

phase shift at said given frequency and-producing sufficient negative feedback in said amplifier tive 'circuitseach havingy frequency determining means tuned` to a different frequency connected between` said output terminals `and said input terminals for causing saidJ amplifier togenerate self-sustained. oscillations of said differentf frequencies; andi amplitude limiting* means in each of said selective circuits'for-limiting the amplitudes `of saidoscillations to: values below the, oyerloadvalue for said amplifier, said,n amplifier comprising a wave amplifying device and a feedback path therefor, said feedback path having substantially zero phase shift for each of said oscillation frequencies and producing suicient negative feedback in said amplifier at each of said oscillation frequencies to render the phase shift from said input terminals to said output terminals substantially 180 degrees at each of said oscillation frequencies.

4. A wave generating and wave amplifying system comprising an amplifier with input terminals and output terminals, a source of waves to be amplified thereby and a load circuit therefor, a plurality of selective circuits connected between said output terminals and said input terminals for causing said amplifier to generate sustained oscillations of chosen different frequencies, each of said selective circuits including a Wheatstone bridge in reactive balance at one of said frequencies and having in one of the bridge arms a resonant impedance tuned to the one frequency, amplitude limiting means individual to each of said selective circuits for limiting the amplitudes of said oscillations to values below the overload value for said amplifier, each of said selective circuits including in series therein between said output terminals and. its amplitude limiting means a series-tuned resonant at the same frequency as its resonant bridge arm impedance for preventing operation of its amplitude limiting means by waves of other frequencies, said amplifier comprising a wave amplifying device and a feedback path therefor, said feedback path having substantially zero phase shift for each of said oscillation frequencies and producing sufficient negative feedback in said amplifier at each of said oscillation frequencies to render the phase shift from said input terminals to said output terminals substantially 180 degrees at each of said oscillation frequencies.

5. An oscillator comprising a wave amplifying device having a feedback circuit forming therewith a closed loop producing self-sustaining feedback of oscillations around said loop and amplitude limiting means in said loop for limiting the amplitude of said oscillations to a value below the overload value for said amplifying device, said amplitude limiting means comprising a thermally sensitive resistance element traversed by said oscillations and varying its temperature and consequently its resistance in response to variations of the oscillation amplitude to maintain substantial constancy of the amplitude, and said resistance element having its mass sufficiently small to render its operating and restoring times required for heating and cooling less than the times required for said oscillations to build up and decay yet having its mass sufliciently large to render its operating and restoring times large compared to the period of said oscillations.

6. The combination with an oscillation generator, of means comprising a thermally sensitive resistance responsive to the generated oscillations to stabilize the amplitude of the oscillations, a temperature control resistor associated with said thermally sensitive resistance for maintaining a constant ambient temperature therefor, and means for supplying energy of the oscillations to said control resistor for heating said control resistor.

7. In combination, a space discharge tube having an input circuit and an output circuit, a feedback coupling for causing said tube to act as a generator of sustained oscillations, a thermally sensitive resistance connected to said tube to stabilize the amplitude of the generated oscillations at a value below that corresponding to the limit of the load-carrying capacity of the tube, whereby a portion of the load-carrying capacity is left unused in the production of the oscillations, means to impress waves, independently of the 0scillations being generated, upon said input circuit to be amplified by said tube and utilized in said output circuit, a temperature control resistor associated with said thermally sensitive resistance for maintaining a fixed ambient temperature therefor, and means for supplying energy of the oscillations to said control resistor for heating said control resistor.

8. A wave translating system comprising a wave amplifying device, an incoming circuit for supplying thereto waves to be selectively amplified by said system, an outgoing circuit for said device for utilizing the selectively amplified waves, a feedback circuit for said device comprising a network having a frequency selective transmission characteristic providing minimum feedback substantially at the frequency to be amplified by said system but strong degenerative feedback at frequencies removed therefrom, a feedback circuit included in said amplifying device for producing negative feedback therein, and means rendering said feedback circuits conjugate to each other.

9. A Wave translating system for selectively amplifying waves of a given frequency to the exclusion of waves of other frequencies, comprising a wave amplifying device, an incoming circuit for supplying thereto waves to be amplified by said system, an outgoing circuit for said device for utilizing the amplified waves, a coupling circuit for feeding waves from the output circuit of said device to the input circuit of said device, a network including in said coupling circuit having a maximum attenuation substantially at said given frequency and relatively 10W attenuation at other frequencies and having its phase shift abruptly change degrees in the neighborhood of said given frequency, a feedback circuit included in said amplifying device for producing negative feedback therein, and means rendering said feedback circuit and said coupling circuit conjugate to each other.

10. A Wave translating system for selectively amplifying waves of a given frequency comprising a wave amplifying device, an incoming circuit for supplying thereto waves to be amplified by said system, an outgoing circuit for said device for utilizing the amplified waves, a coupling circuit for feeding waves from the output circuit of said device to the input circuit of said device, a Wheatstone bridge network included in said coupling c circuit and having in one arm an impedance resonant substantially at said give frequency, said bridge network being adjusted for a maximum attenuation substantially at said given frequency and relatively low attenuation at other frequencies, a feedback circuit included in said amplifying device for producing negative feedback therein, means rendering said feedback circuits conjugate to each other, and means rendering said coupling circuit and said outgoing circuit conjugate to each other.

HAROLD S. BLACK. 

